I thought I share a little VCF-circuit I've been working on.
It's a pretty simple circuit using just a single opamp, some resistors two caps and a diode. I'm pretty sure someone else already had the same idea but I still think it's worth sharing.

It's not a very accurate filter but it can be powered from a single 9V battery. And for those who are looking for cheap and simple VCFs this might be the solution (or at least a pointer to the right direction).

I ran some tests with some drum- and synthloops.
It should be ok especially for noise boxes, drones and other crazy and noisy stuff

Enough talk!

Here is the schematic and a little recording with a drumloop and a version with an additional synth.

I lowered to summing resistors on purpose.
They were 100k but that didn't gave me the filter sweeps I wanted - so I lowered them down. The circuit itself has also changed. But it's not finished yet. I'm still trying to improve the frequency range.

The filters you posted look pretty cool. I think I'll have to breadboard them.
My Idea for this filter came from some simple, passive VCF circuits which didn't offer Resonance control. So I tried to include the technique used there in an active design.
I was pretty sure that I came up with something new
But as I already mentioned: I'm sure that someone else had the same Idea.

Had the same issue with a 2 op amp LFO I thought I designed.
But I realized that it was such a simple standard design - it has been posted several times in many forums. So no post here.

I'm currently building a little filter toy with my design. Perhaps the filters you posted work out better. I will give it a try.

Thx for the hint! _________________"I don't care much about music. What I like is sounds."

This is an active bridged-T filter.
Just google for bridged-T filter and you should find the equations. I selected the values by trial and error

To improve the frequency range you could exchange the diode with a garden variety transistor. e.g. BC547.

Connect the emitter to the junction of the two caps and the collector to ground (0V). Apply the CV to base. This should improve the behaviour to cv and give you some more control - especially with low CVs. You should then rise the summing resistor values to at least 100k. To limit the filter's frequency response when no CV is applied you could try to place a resistor in parralel to collector and emitter.

I think this is the same way Ray Wilson achieved a VCF in one of his designs. I'm pretty sure I saw this method in one of his old devices. I came up with this when I modified my WSG. Now it has some nice "quasi-vcf"

Hope this helps a little.

Greetz_________________"I don't care much about music. What I like is sounds."

Thanks. I ended up doing the same, using a transistor, on the advice from people here, and it works great ! I tried using 100k resistors for the CVs, without much success I'm afraid. I had very little range on the controls. So back to 10k. Putting a resistor across the transistor is great too, actually it answers an issue I had concerning the pots' travel…
So. Besides, I had put this transistor the other way round, like I thought it should have to be (as I mentioned on your VCO topic). The fact is that both work. I won't even ask questions here. I just don't understand.
By the way, your filter is among others discussed here._________________::U::N::S::C::H::N::E::L::L::

i tried to improve the sweep and cv following the tips here on this thread, but didn't worked...i would be also curious to see new schematics
i'm trying to use this filter to build a baby 8 for a friend, with internal vco and vcf controlled by the sequencer
kind of self contained groovebox
pretty funny till now, but sure could be even better
when i feed the signal to the filter the volume goes drastically down
and even with the cutoff completely open, i don't have the original sfrequency spectrum of the original, i loose part of the bottom and start to sound almost like an hipass
plus the pots don't have a linear response, too less and then too much if u know what i mean, should i try with a couple of logs ?

anyway the sound is already more interesting than the wsg filter for example, at least the resonance is not only perceived

Replace R1 with a 100k resistor. This should fix the volume drop problem without (drastically) affecting the operation of the filter. You could even go lower to compensate a bit more if needed. Note that this will lower the input impedance of the filter, so make sure the output of the previous stage has a low enough output impedance, or you could get attenuation from the impedance mismatch (this shouldn't generally be a problem unless you have a volume pot before the filter or something like that)

I'm not sure why R1 is 470k in the first place. It looks like maybe a mistake? Or maybe the attenuation was intentional? Sorry I'm new here.

I made a similar filter, using an LED and a 47K resistor, I got quite a good range from it but it was very unstable bursting into HF oscillation and causing the output to clip. Is this an inherently unstable circuit? Perhaps I need better supply decoupling...

The polarized capacitor you have on your LED might be to blame, if you get oscillation you may need to adjust the resonance control or input amplitude._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Hey all, first post here, slapped this VCF together and works quite nice, but i have a problem with hiss and noise with cutoff and resonance set low (as in, cutoff completely open, and minimal resonance). At frst I thought it was ceramic caps for those two 100nF, but swapped them for film ones and the noise is only a littke better. I used 10k resistors for CV, and 100k for input, there is also a BC546 instead of the diode. Should I trim the input signal more? Or maybe it has to be like this? I used the schematic at the top of this thread.

What op amp are you using? people usually blame op amps but maybe you need some high pass filtering at the input/output?
Also your connections between the circuit and amplifier, or whatever you are listening to it with, could be sheilded poorly, you could even be in an electromagnetic field hot spot _________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

op amp - TL071cn , i used normal guitar cables and a small guitar amp. In the meantime I tried it during a band practice, and we are so loud we barely hear that noise. It would be nice however, to have it clean for studio purposes. Also, what voltage swing from an LFO should I use to modulate it from full to no cutoff, 0-9V?

There is another thread about simple VCFs somewhere else on this forum but I don't know what it was called anymore (I think umschmitt posted there too), anyway a very similar circuit was used but with a transistor in place of the diode, this should help with noise problems and allow you to tailor the control voltage range._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

I've just built this circuit using the original circuit at the top, only exception being that I use a tl072 instead of the tl071, but I guess that shouldn't matter.

nope.

Quote:

My first guess is that is because of the weak signal coming from the bass guitar.

yup.

as elmigil has pointed out, this circuit would require a preamp from anything that has a low output such as a bass guitar. you might want to look up a simple high gain op amp circuit, you could even use the other half of the tl072.

Quote:

I have the same problem with the "white noise" that my VCF produce

HELP!

firstly to electro-music
and we will need a little more information, what signal are you putting into this, how is it powered? have you altered any of the component values?
information is power __________________Richard_

This may be a really idiotic comment, but I can't seem to find any mention of screened connections to the input and output jacks, which I would do as a matter of course. As I say, this may be stating the "bleedin' obvious" but it depends on how new you are to electronics. I certainly would not have thought of it when I first started out.

Hi! my VCF is powered by 9v comming from a wall wart adapter, my incomming signal is a Atari Punk Console powered by a 9v battery.

whoah, I totally lost this thread. the noise is likely as analog_backlash said, a grounding issue, you have 2 separate power supplies there, if you do this you should "common" the grounds, so just run a bit of wire from the 0v of the battery to the 0v of the wall-wart__________________Richard_

You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot vote in polls in this forumYou cannot attach files in this forumYou can download files in this forum

Please support our site. If you click through and buy from our affiliate partners, we earn a small commission.